Imagine I have a PCR reaction sample with a volume of 20 Ál. After the
PCR reaction I want to put it onto a gel, so I need blue marker loading dye.
My blue marker loading dye is 6x concentrated so I would obviously take 4 Ál
(4 Ál of a 6x buffer in 24 Ál volume then). Everything is fine then.

But now I am wondering how I can calculate the volume which would be required if
I used 7x buffer and got stuck in a caclulation...

1 x (20+X) = X x 7 (X = the volume of the buffer which is required)

It is obvious that it should be something between 3 and 4, but how can I calculate
it exactly?